1. Field of the Invention
The present invention relates to a rubber composition which is excellent in heat aging resistance.
2. Description of the Related Art
Heat aging of rubber is caused according to an auto-oxidation mechanism of polymers which is generally known, and therefore it is effective for preventing heat aging to inactivate peroxy radicals produced in this course.
In general, diphenyldiamine based compounds and hindered phenol based compounds have so far been used as an antioxidant. These compounds give xe2x80x9cHxe2x80x9d in xe2x80x9c greater than NHxe2x80x9d and xe2x80x9cxe2x80x94OHxe2x80x9d to peroxy radicals produced in the course of auto-oxidative degradation to inactivate them, and the compounds themselves change to stable compounds through more stable radicals. Thus, widely known is a method in which peroxy radicals are inactivated to terminate a radical chain reaction to thereby prevent aging.
Both the conventional diphenyldiamine based antioxidants and hindered phenol based antioxidants described above have an aging preventing action. In general, as the blending amount thereof is increased, an aging preventing effect grows large correspondingly in an area where the blending amount is small, but when they are used in a large amount, an increment in the effect becomes gradually small. The effect is even reduced due to bloom in a certain case, and therefore involved is the problem that the optimum use amount has to be set.
Further, a rubber composition using a diphenylamine based antioxidant has a large change rate in an elastic modulus before and after degradation, and the rubber composition is notably hardened and reduced in its physical properties in a certain case depending on the blending recipe of the rubber composition and the conditions of degradation.
In light of the problems of the conventional techniques described above, an object of the present invention is to solve the problems and to provide a rubber composition which is excellent in heat aging resistance without causing a reduction in physical properties.
Intensive researches repeated by the present inventors in order to solve the problems of the conventional technique described above have resulted in successfully obtaining a rubber composition meeting the object described above by blending a specific amount of a specific compound with a rubber component comprising at least one selected from the group consisting of natural rubber and synthetic rubbers, and thus the invention has been completed.
That is, the rubber composition of the present invention relates to the following compositions (1) to (8):
(1) A rubber composition comprising 100 parts by weight of a rubber component comprising at least one selected from the group consisting of natural rubber and synthetic rubbers and 0.05 to 20 parts by weight of at least one selected from compounds (A) represented by the following Formula (I): 
wherein A1 and A2 each represent Rxe2x80x94, Rxe2x80x94NHxe2x80x94, Rxe2x80x94Xxe2x80x94NHxe2x80x94, Rxe2x80x94NHxe2x80x94NHxe2x80x94 or Rxe2x80x94Xxe2x80x94NHxe2x80x94NHxe2x80x94, and A1 and A2 may be the same or different; R represents any one of a hydrogen atom, an alkyl group having 1 to 18 carbon atoms which may be branched, a cycloalkyl group, an aryl group, an alkylaryl group or an alkenyl group, each group of which may include at least one substituent containing a sulfur atom, a nitrogen atom and an oxygen atom, and two R""s may be combined in a molecule to form a ring containing a  greater than Cxe2x95x90NH bond; and X represents any one of those shown below: 
(2) The rubber composition as described in the above item (1), wherein the compound (A) represented by Formula (I) described above is a compound represented by the following Formula (II): 
wherein B1 and B2 each represent: 
a hydrogen atom or an alkyl group having 1 to 18 carbon atoms which may be branched, and B1 and B2 may be the same or different.
(3) The rubber composition as described in the above item (2), wherein the compound represented by Formula (II) is Nxe2x80x2-(N-benzoylamidino)benzoic acid hydrazide.
(4) The rubber composition as described in the above item (3), wherein the compound (A) represented by Formula (I) described above is a compound represented by the following Formula (III): 
wherein B1 and B2 each represent: 
a hydrogen atom or an alkyl group having 1 to 18 carbon atoms which may be branched, and B1 and B2 may be the same or different.
(5) The rubber composition as described in the above item (4), wherein the compound represented by Formula (III) is N-amidinobenzamide.
(6) The rubber composition as described in any of the above items (1) to (5), further comprising an antioxidant, wherein the antioxidant is at least one selected from the group consisting of antioxidants of a naphthylamine base, a p-phenylenediamine base, a hydroquinone derivative, a monophenol base, a bisphenol base, a trisphenol base, a polyphenol base, a diphenylamine base, a quinoline base, a thiobisphenol base and a hindered phenol base.
(7) The rubber composition as described in the above item (6), wherein the antioxidant described above is at least one selected from the group consisting of antioxidants of a p-phenylenediamine base and a diphenylamine base.
(8) The rubber composition as described in the above item (5) or (6), wherein an amount of the antioxidant described above is 0.1 to 5.0 parts by weight per 100 parts by weight of the rubber component.
A rubber composition which is better in heat aging resistance is provided by using at least one selected from the compounds (A) represented by Formula (I) in combination with an antioxidant.
The present invention shall be explained below in detail.
The rubber composition of the present invention is characterized by comprising 100 parts by weight of a rubber component comprising at least one selected from the group consisting of natural rubber and synthetic rubbers and 0.05 to 20 parts by weight of at least one selected from the compounds (A) represented by the following Formula (I): 
wherein A1 and A2 each represent Rxe2x80x94, Rxe2x80x94NHxe2x80x94, Rxe2x80x94Xxe2x80x94NHxe2x80x94, Rxe2x80x94NHxe2x80x94NHxe2x80x94 or Rxe2x80x94Xxe2x80x94NHxe2x80x94NHxe2x80x94, and A1 and A2 may be the same or different.
Above R is any one of a hydrogen atom, an alkyl group having 1 to 18 carbon atoms which may be branched, a cycloalkyl group, an aryl group, an alkylaryl group or an alkenyl group (these groups include a group having at least one substituent containing a sulfur atom, a nitrogen atom and an oxygen atom), and two R""s may be combined in a molecule to form a ring containing a  greater than Cxe2x95x90NH bond.
X represents any one of those shown below: 
The compound (A) represented by Formula (I) used in the present invention is used as a novel heat aging preventive. Although an action mechanism thereof is not sufficiently made clear, it is considered that xe2x80x9c greater than NHxe2x80x9d is contained in the structure thereof as is the case with conventional diphenylamine base antioxidants and therefore gives a proton to peroxy radicals produced in the course of an auto-oxidation of the rubber composition to stabilize, whereby the aging preventing action is displayed (this point shall further be explained in examples described later).
The compound (A) represented by Formula (I) includes, for example, various compounds shown in the following items (1) to (14).
(1) When A1 and A2 are Rxe2x80x94, included are, for example, ethaneimine, benzylimine, acetoneimine, acetophenoneimine and benzophenoneimine.
(2) When A1 is Rxe2x80x94 and A2 is Rxe2x80x94NHxe2x80x94, included are, for example, benzoamidine, acetoamidine, 1-phenylbenzoamidine, 1-methylbenzoamidine and 1-phenylacetoamidine.
(3) When A1 is Rxe2x80x94 and A2 is Rxe2x80x94Xxe2x80x94NHxe2x80x94, included are, for example, 1-benzoylbenzoamidine, 1-acetylbenzoamidine and 1-benzoylacetoamidine.
(4) When A1 and A2 are Rxe2x80x94NHxe2x80x94, included are, for example, guanidine, 1-phenylguanidine, 1-methylguanidine, 1,3-diphenylguanidine, 1,3-dipropylguanidine and 1-phenyl-3-methylguanidine.
(5) When A1 is Rxe2x80x94NHxe2x80x94 and A2 is Rxe2x80x94Xxe2x80x94NHxe2x80x94, included are, for example, N-amidinobenzamide, N-amidinoacetoamide, Nxe2x80x2-(N-phenylamidino)benzamide, Nxe2x80x2-(N-phenylamidino)acetoamide, Nxe2x80x2-(N-phenylamidino)stearamide, Nxe2x80x2-(N-methylamidino)benzamide and biguanide.
(6) When A1 and A2 are Rxe2x80x94Xxe2x80x94NHxe2x80x94, included are, for example, 1,3-dibenzoylguanidine, 1,3-diacetylguanidine and 1-acetyl-3-benzoylguanidine.
(7) When A1 is Rxe2x80x94 and A2 is Rxe2x80x94NHxe2x80x94NHxe2x80x94, included are, for example, benzohydrazideimine, acetohydrazideimine, 2-phenylbenzohydrazideimine, 2-methylbenzohydrazideimine and 2-phenylacetohydrazideimine.
(8) When A1 is Rxe2x80x94 and A2 is Rxe2x80x94Xxe2x80x94NHxe2x80x94NHxe2x80x94, included are, for example, 2-benzoylbenzohydrazideimine, 2-acetylbenzo-hydrazideimine, 2-benzoylacetohydrazideimine and 2-amidinobenzohydrazideimine.
(9) When A1 is Rxe2x80x94NHxe2x80x94 and A2 is Rxe2x80x94NHxe2x80x94NHxe2x80x94, included are, for example, aminoguanidine, 1-amino-2-phenylguanidine, 1-amidino-2-phenylhydrazine, 1-phenyl-2-(N-phenyl-amidino)hydrazine, 1-methyl-2-(N-phenylamidino)hydrazine and 1-phenyl-2-(N-methylamidino)hydrazine.
(10) When A1 is Rxe2x80x94NHxe2x80x94 and A2 is Rxe2x80x94Xxe2x80x94NHxe2x80x94NHxe2x80x94, included are, for example, Nxe2x80x2-amidinobenzoic acid hydrazide, Nxe2x80x2-amidinooctanoic acid hydrazide, Nxe2x80x2-(N-phenylamidino)benzoic acid hydrazide, Nxe2x80x2-(N-methylamidino)benzoic acid hydrazide, Nxe2x80x2-(N-phenylamidino)acetohydrazide and Nxe2x80x2-(N-phenylamidino)octanoic acid hydrazide.
(11) When A1 is Rxe2x80x94Xxe2x80x94NHxe2x80x94 and A2 is Rxe2x80x94Xxe2x80x94NHxe2x80x94NHxe2x80x94, included are, for example, Nxe2x80x2-(N-benzoylamidino)benzoic acid hydrazide, Nxe2x80x2-(N-octanoylamidino)octanoic acid hydrazide, Nxe2x80x2-(N-acetylamidino)benzoic acid hydrazide, Nxe2x80x2-(N-benzoylamidino)propionic acid hydrazide, Nxe2x80x2-(N-benzoylamidino)octanoic acid hydrazide and Nxe2x80x2-(N-benzoylamidino)stearic acid hydrazide.
(12) When A1 and A2 are Rxe2x80x94NHxe2x80x94NHxe2x80x94, included are, for example, carbohydrazideimine, 1,5-diphenylcarbo-hydrazideimine and 1,5-dimethylcarbohydrazideimine.
(13) When A1 is Rxe2x80x94NHxe2x80x94NHxe2x80x94 and A2 is Rxe2x80x94Xxe2x80x94NHxe2x80x94NHxe2x80x94, included are, for example, 1-benzoyl-5-phenylcarbohydrazideimine, 1-acetyl-5-phenylcarbohydrazideimine and 1-benzoyl-5-methylcarbohydrazideimine.
(14) When A1 and A2 are Rxe2x80x94Xxe2x80x94NHxe2x80x94NHxe2x80x94, included are, for example, 1,5-dibenzoylcarbohydrazideimine, 1,5-diacetylcarbohydrazideimine and 1,5-dioctanoylcarbo-hydrazideimine.
In the various compounds of the items (1) to (14) described above, those in which structural double bond isomers are present shall not be restricted only to one isomer.
These compounds (A) can be used alone or in a mixture of two or more kinds thereof.
The compound (A) represented by Formula (I) described above used in the present invention is preferably the compound represented by the following Formula (II) or (III): 
wherein B1 and B2 each represent: 
a hydrogen atom or an alkyl group having 1 to 18 carbon atoms which may be branced, and B1 and B2 may be the same or different: 
wherein B1 and B2 each represent: 
a hydrogen atom or an alkyl group having 1 to 18 carbon atoms which may be branched, and B1 and B2 may be the same or different.
To be specific, the compound (A) is preferably Nxe2x80x2-(N-benzoylamidino)benzoic acid hydrazide in Formula (II) and N-amidinobenzamide in Formula (III). In these cases, better heat aging preventive action is displayed.
These compounds (A) can be relatively easily synthesized by reacting readily available raw materials such as ketones, amines, nitrites, hydrazines, hydrazides, guanidines, aminoguanidines and iminoethers with each other, or reacting these compounds with suitable reactants. The specific synthetic examples shall further be explained in examples.
A content of at least one of the compounds (A) represented by Formula (I) described above used in the present invention is 0.05 to 20 parts by weight, preferably 0.1 to 5.0 parts by weight and more preferably 0.5 to 3.0 parts by weight per 100 parts by weight of the rubber component.
If the amount of the compound (A) described above falls in a range of 0.05 to 20 parts by weight, the intended heat aging preventing effect is exhibited without bringing about a reduction in the other physical properties and a rise in the cost.
In the present invention, at least one of the various compounds described above is used in combination with an antioxidant which is conventionally used in the rubber industry, whereby better heat aging preventing action is displayed.
It has so far been known that when two or more kinds of antioxidants are used in combination, a synergistic effect is exhibited depending on the combination of the antioxidants used, and it is widely known that the kind of this synergistic effect includes homosynergism which takes place when two or more kinds of antioxidants themselves of a peroxy radical scavenger type are used in combination, and heterosynegism which takes place when antioxidants having different action mechanisms are used in combination, for example, when antioxidants of a peroxy radical scavenger type are combined with antioxidants of a peroxide decomposer type. In the present invention, various kinds of the preceding compounds (A) are, as described above, the antioxidants of a peroxy radical scavenger type, and when they are used in combination with other conventional antioxidants of a peroxy radical scavenger type such as a diphenyldiamine base and a hindered phenol base, the former homosynergism is revealed. As a result, displayed is the specific effect that the larger effect is obtained in a smaller amount in the case of using the compound (A) in combination with the conventional antioxidants of a diphenyldiamine base and a hindered phenol base rather than in the case of using the conventional antioxidants alone in a large amount.
Rubber composition using this compound (A) is less deteriorated in the elastic modulus before and after degradation than rubber composition blending a diphenyldiamine based antioxidant.
Given as the antioxidants used in combination are a naphthylamine base, a p-phenylenediamine base, a hydroquinone derivative, a monophenol base, a bisphenol base, a trisphenol base, a polyphenol base, a diphenylamine base, a quinoline base, a thiobisphenol base and a hindered phenol base. Among them, amine based antioxidants of a p-phenylenediamine base and a diphenylamine base are preferred in terms of the higher aging preventing effect.
The antioxidants of a p-phenylenediamine base include, for example, N,Nxe2x80x2-diphenyl-p-phenylenediamine, N-isopropyl-Nxe2x80x2-phenyl-p-phenylenediamine, N,Nxe2x80x2-di-2-naphthyl-p-phenylenediamine, N-cyclohexyl-Nxe2x80x2-phenyl-p-phenylenediamine, N-phenyl-Nxe2x80x2-(3-methacryloyloxy-2-hydroxypropyl)-p-phenylenediamine, N,Nxe2x80x2-bis(1-methyl-heptyl)-p-phenylenediamine, N,Nxe2x80x2-bis(1,4-dimethylpentyl)-p-phenylenediamine, N,Nxe2x80x2-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine and N-(1,3-dimethylbutyl)-Nxe2x80x2-phenyl-p-phenylenediamine. Among them, N-(1,3-dimethylbutyl)-Nxe2x80x2-phenyl-p-phenylenediamine is most preferred in terms of the higher aging preventing effect.
The antioxidants of a diphenylamine base include, for example, 4,4xe2x80x2-(xcex1-methylbenzyl)diphenylamine, 4,4xe2x80x2-(xcex1,xcex1-dimethylbenzyl)diphenylamine, p-(p-toluene-sulfonylamide)diphenylamine and 4,4xe2x80x2-dioctyldiphenyamine. Among them, 4,4xe2x80x2-(xcex1-methylbenzyl)diphenylamine is most preferred in terms of the higher aging preventing effect.
The antioxidants used in combination with the compound (A) represented by Formula (I) described above can be used alone or in combination of two or more kinds thereof, and they are used in a range of 0.1 to 5.0 parts by weight, preferably 0.2 to 3.0 parts by weight per 100 parts by weight of the rubber component.
When a content of the compound described above falls in a range of 0.1 to 5.0 parts by weight, the higher aging preventing effect is exhibited without causing bloom on a surface of a rubber article and a reduction in other physical properties.
The rubber component used in the present invention is natural rubber and/or synthetic rubbers. The synthetic rubbers include, for example, cis-1,4-polyisoprene, styrene-butadiene copolymers, 1,4-polybutadiene, ethylene-propylene-diene copolymers, polychloroprene, halogenated butyl rubber and acrylonitrile-butadiene rubber. At least one of them can be used, and natural rubber or diene base synthetic rubber is preferably used.
In the present invention, capable of being suitably blended, if necessary, in addition to the compounds described above are compounding ingredients usually used in the rubber industry, for example, reinforcing fillers such as carbon black and silica, process oil, other antioxidants than the antioxidants described above, vulcanization-accelerators, acceleration activators such as stearic acid and zinc white, and vulcanizing agents such as sulfur.
The reinforcing fillers include, for example, carbon black, silica, calcium carbonate and titanium oxide, and among them, carbon black is preferred. One usually used in the rubber industry can suitably be used as carbon black. In particular, carbon black of HAF, ISAF or SAF grade is preferred.
An amount of the reinforcing filler is preferably 20 to 150 parts by weight per 100 parts by weight of the rubber component.
The process oil which can be used includes, for example, a paraffin base, a naphthene base and an aromatic base.
The vulcanization-accelerators which can be used shall not specifically be restricted and include thiazoles such as mercaptobenzothiazole and dibenzothiazyldisulfide, sulfenamides such as N-cyclohexyl-2-benzothiazolylsulfenamide, N,Nxe2x80x2-dicyclohexyl-2-benzothiazolylsulfeneamide and Nxe2x80x2-t-butyl-2-benzothiazolyl-sulfenamide, and guanidines such as diphenylguanidine. The blending amount thereof is preferably 0.1 to 5 parts by weight per 100 parts by weight of the rubber component.
The acceleration activators shall not specifically be restricted and include stearic acid and zinc white.
Vulcanizing agents usually used in the rubber industry can suitably be used and include sulfur and peroxides, and sulfur is preferred. A blending amount of the vulcanizing agent is preferably 0.5 to 8.0 parts by weight, more preferably 0.5 to 5.0 parts by weight per 100 parts by weight of the rubber component. When the amount of the vulcanizing agent falls in a range of 0.5 to 8.0 parts by weight, vulcanization is sufficiently carried out. In addition thereto, the scorching time can be extended, and rubber does not get scorched during kneading.
The rubber composition of the present invention can be used for industrial articles such as rubber vibration insulators, belts and hoses as well as tire materials such as tire treads, under treads, carcasses, side walls and beads. It is used preferably for tire treads.
Products including tires, power belts and the like are envisioned. Decreased rolling resistance is, of course, a useful property for pneumatic tires, both radial as well as bias ply types and thus, the vulcanizable elastomeric compositions of the present invention can be utilized to form treadstocks for such tires. Pneumatic tires can be made according to the constructions disclosed in U.S. Pat. Nos. 5,866,171; 5,876,527; 5,931,211; and 5,971,046, the disclosures of which are incorporated herein by reference. The composition can also be used to form other elastomeric tire components such as subtreads, black sidewalls, body ply skims, bead fillers and the like.